Musculoskeletal injuries have a substantial impact on the health and quality of life of millions of Americans. Delayed healing of and non-unions of fractures represent a continuous orthopaedic challenge. The conventional way of treating these problems is to use bone plates or screws in combination with autologous bone grafting.
As a natural composite material, autogenous bone graft has been shown to have both osteoconductive and osteoinductive properties. In addition, it is a sterile, non-immunogenic and non-toxic material, which has the ability to be fully incorporated into the fracture site. Notwithstanding the long duration for their activity to develop, autogenous bone grafts are the gold standard by which synthetic composites are compared. Given that there is also a limited supply and harvest site morbidity of autogenous bone graft material, there is significant motivation to develop synthetic composites. To date, no synthetic bone graft substitutes have fully achieved the properties of autogenous bone graft.
Enhancing the rate and probability of fracture healing and the promotion of bone formation and healing of delayed and non-union fractures are of great clinical significance. (NIH/AAOS sponsored workshop. Bone Formation and Bone Regeneration. Tampa, Fla.: American Academy of Orthopaedic Surgeons, 1993). The large population of patients with delayed unions and non-unions of bone, the large direct medical costs, and the societal costs related to their long term disability, highlight the need for effective and improved methods of treatment.
Advances in materials science and the identification of osteogenic and osteoinductive growth factors have invited the investigation of newer alternatives for autogenous bone grafting. Osteogenesis, which is the process of bone formation, involves both osteoconduction and osteoinduction. Osteoconduction is the process in which differentiated bone-forming cells produce a bone matrix upon an existing substrate. Materials that promote this process are considered osteoconductive. Osteoinduction is the process by which undifferentiated mesenchymal precursor cells are transformed into differentiated bone forming cells. Factors or materials that promote this process are considered to be osteoinductive.
Growth factors delivered by biologically active controlled release carriers have the potential for improved fracture healing and lower morbidity, thereby resulting in improved patient care and a decrease in the overall costs associated with fracture care. Similarly, the delivery of antibiotics by such carriers, either alone or in addition to growth factors, will help reduce the incidence of infections, which can further contribute to delays in healing. In fractures involving, for example, the spine, the incorporation of anti-inflammatory agents and analgesics will help control inflammation, which can also delay the healing process, and contribute to patient comfort during the healing process. Additionally, the controlled release of such materials regardless of the bioactivity of the carrier would represent a distinct advantage over current delivery methods and assist fixation of implants.
The ideal synthetic graft would be a scaffolding material that would stimulate bone tissue to grow in place of the scaffold as it degrades. (Damien et al., J. Applied Biomater. (1991) 2:187-20.) Synthetic materials intended as bone graft substitutes should have mechanical and other properties similar to those of bone, and should be biocompatible with the surrounding tissues. In order to provide a union across the fracture site they must serve not only as scaffolding materials but also, similarly to native bone, have a stimulatory effect on bone tissue regeneration.
The currently used synthetic bone graft materials are considered osteoconductive in that they elicit the formation of the bone matrix at their surfaces. Furthermore, they lead to a contiguous interface with bone or are replaced by bone tissue. Such properties suggest a chemical interaction between these bioactive materials and the bone environment. Cells existing in the bone matrix environment exhibit a beneficial response to these materials.
The materials studied most for use as synthetic grafts have been calcium phosphate ceramics and bioactive glasses. Calcium phosphate ceramics (CPCs) are very similar in composition to the mineral phase of bone. Bioactive glass are capable of forming a hydroxyapatite layer on their surface that mimics the mineral phase of bone.
The most commonly used calcium phosphate ceramics include: hydroxyapatite (HA), in either dense or porous forms, and .beta.-tricalcium phosphate (.beta.-TCP). Hydroxyapatite is of limited effectiveness as a grafting material. When HA particulate material in porous and dense form was evaluated as a grafting material in the alveolar ridge it was found that fibrous encapsulation formed in perosseous sites. Migration of the particles was also found to be a problem. (Ducheyne P., J. Biomed. Mater. Res. (1987) 21(A2 Suppl):219.) Further, HA cannot be used as a scaffolding material since its rate of degradation is slow. (Cornell et al., Clin. Orthop. (1992) 297; and Radin et al., J. Biomed Mater. Res. (1993) 27:35-45.)
.beta.-TCP, on the other hand, is a biodegradable material which is osteoconductive. However, its degradation rate has been found to be too fast to serve as an effective synthetic graft material in load-bearing situations. (Damien et al., supra.) Thus, clinical evaluations and applications of the HA and .beta.-TCP materials, either dense or porous, have demonstrated that both materials are limited by a lack of controlled rate of reactivity.
Bioactive glasses were first found to bond to living bone by Dr. Larry Hench in the late 1960's. Since that time, more than ten groups around the world have shown that glasses containing SiO.sub.2, CaO, P.sub.2 O, Na.sub.2 O and other smaller amounts of oxides in various compositions bond to bone. (Ducheyne P., J. Biomed Mater. Res. (1987) 21(A2 Suppl):219; Hench, L. L., Ann. N.Y. Acad. Sci. (1988) 523:54; Andersson et al., J. Biomed Mater. Res. (1991) 25:1019-1030; Andersson et al., J. Non-Cryst Solids (1991) 129:145-151; Boone et al., J. Biomed Mater. Res. (1989) 23(A2 Suppl):183; Ducheyne et al., Clin. Orthop. Rel. Res. (1992) 76:102-114; Hench, L. L., J. Biomed Mater. Res. (1989) 23:685-703; Kokubo, T., Biomaterials (1991) 12(2):155; and Rawlings, R. D., J. Mater. Sci. Letters (1992) 11:1340-1343.)
Bioactive glass-ceramics undergo surface corrosion reactions when exposed to body fluids. These corrosion reactions form a silica-rich surface layer. This layer serves as a nucleation site for the deposition of calcium phosphate, which evolves into a thick hydroxyapatite layer. When in contact with bone forming cells, this layer will form the basis of the chemical bond between the glass and the bone matrix. (Ducheyne, supra; Hench (1988), supra; and Hench, (1989), supra.) Dr. Hench's 45S5 bioactive glass has been the most extensively studied of the bioactive glass-ceramics. Its composition by weight % is: 45% SiO.sub.2, 24.5% CaO, 6% P.sub.2 O.sub.5 and 24.5% Na.sub.2 O.
In U.S. Pat. No. 5,204,106 (incorporated herein by reference), 45S5 glass in particulate form in a narrow size range was described as being an effective bone graft substitute in the alveolar ridge model and as being well incorporated into the surrounding bone. The glass granules were described as causing the upregulation of osteoprogenitor cells to osteoblasts that actively lay down bone tissue. (Schepers et al., J. Oral Rehabil. (1991) 18:439-452.).
The following parameters are important for bone-bioactive synthetic grafts: controlled resorption and reactivity, immersion induced transformation of the synthetic materials' surface into a biologically-equivalent hydroxyapatite-like mineral, relatively large surface area, and porosity (to create a network for osteoblastic activity). Bioactive glass can potentially be tailored to fit these parameters. In addition, the following requirements are important for a successful delivery system for biologically active molecules: 1) controlled release of the molecules; 2) delivery of adequate amounts of the molecules; 3) rapid growth of bone tissue into the carrier; 4) biocompatibility, osteoconductivity, and osteoinductivity of the implant material; and 5) resorption of the carrier once bone tissue has completely formed. (Lucas et al., J. Biomed Mater. Res. (1989) 23(A1 Suppl):23.) No delivery system currently available meets all of these criteria. (Damien et al., supra; and Cornell and Lane, Clin. Orth. Rel. Res. (1992) 277:297-311.) Certainly, no delivery system results in controlled delivery.
Attempts have been made to try to improve calcium phosphate ceramics by using them as delivery vehicles for bone growth factors. To date, there has been no success in incorporating growth factors into calcium phosphate ceramics in a way that will lead to a sustained release of the added growth factor. Mostly, one achieves a "burst" release, which is a rapid initial release of most of the material over a short period of time. (Campbell et al., Trans. Orthop. Res. Soc., 40:775, 1994.)
Carriers made of .beta.-TCP, or nonsoluble collagen, have been moderately successful when combined with bone morphogenetic protein in attaining good acceleration of bone tissue healing. (Damen et al., J. Dental Res. (1989) 68:1355-1359.) However, these systems have not been able to produce a measurable, controlled release of growth factor for time spans approaching those needed for bone tissue regeneration to span large bone filling defects. In one study, large amounts of growth factors, i.e. greater than 50 milligrams, were required to fill defects greater than three (3) centimeters. (Johnson et al., Clin. Orthop. (1992) 277:229-237.)
In most of the systems studied with osteoconductive materials used as carriers, the method of incorporation has been that of simple immersion of the material into a growth factor solution. The growth factor is then adsorbed either onto the material surface or into the pore structure, but is then quickly released upon immersion in an aqueous solution in a burst effect. (Campbell et al., Trans. Orthop. Res. Soc., 40:775, 1994.)
Published application WO 92/07554 reports a material which can be implanted in living tissue which has a biodegradation rate matching the rate at which the tissue regenerates. It is reported that the material may include an active substance providing an extended therapeutical effect. The material includes a calcium phosphate, biodegradable oxide or polyoxide, and an active substance having amine groupings such as netilmicin and/or gentamicin in sulphate form.
Published application WO 93/05823 reports a composition for stimulating bone growth comprising at least one of FGF, TGF-.beta., IGF-II, PDGF, and their biologically active mutants and fragments, or bone extracts with corresponding activity, or bone extracts with BMP activity, and a suitable application material.
United Kingdom Patent Application GB 2255907 A reports a delivery system for biologically active growth and morphogenetic factors comprising a solid adsorbent selected for its specific affinity for the factor and the factor adsorbed thereon. In one embodiment, porous hydroxyapatite is specified as the solid adsorbent.
U.S. Pat. No. 4,869,906 describes a resorbable porous tricalcium phosphate in which the pores are sealed with a filler mixture of antibiotic and a filler.
U.S. Pat. Nos. 5,108,436 and 5,207,710 describe stress-bearing prostheses having a porous region in combination with an osteogenic factor extract or a purified osteogenic inductive protein, optionally in combination with a TGF-.beta. cofactor, in a pharmaceutically acceptable carrier. The carrier is either a collagen composition or a ceramic. The osteogenic factor extract is dispersed in the porous region. Other procedures for combining the stress-bearing member with the osteoconductive material including coating, saturation, applying vacuum force to get the material into the pores, and air-drying or freeze-drying the material onto the member. It is further described that the pharmaceutically acceptable carriers preferably include a matrix that is capable of providing a structure for developing bone and cartilage. Some preferred pharmaceutically acceptable carriers listed include collagen, hydroxyapatite, tricalcium phosphate, and bioactive glass. However, there is no description of a preparation containing bioactive glass as a pharmaceutically acceptable carrier.
U.S. Pat. No. 4,772,203 describes implants having a core and a matrix, with the matrix being at least partially resorbable. The resorbable matrix is one or both of bioactive and osteogenesis-inducing. Tricalcium phosphate, hydroxylapatite sic!, and bioactive glass are listed as such matrixes. It is further stated that if a resorbable matrix is employed, it is further possible to embed antibiotics in the latter.
U.S. Pat. No. 4,976,736 describes biomaterials useful for orthopedic and dental applications having a base portion of calcium carbonate and a surface layer of a synthetic phosphate such as hydroxyapatite. One advantage asserted for hydroxyapatite is absorbency. It is further described that antibiotics or growth factors can be introduced into the pore cavities of the implant or attached, respectively. Alternatively, the antibiotic or growth factor can be intermixed with a preferably biodegradable polymer and injected or vacuum infiltrated into the porosity of the phosphate surfaced material.
Gombotz et al., J. App. Biomat., (1994) 5:141-150 describe the incorporation of transforming growth factor-.beta. into a composite implant made from poly(lactic-co-glycolic acid) and demineralized bone matrix. It is reported that the implants exhibited an inflammatory response with little mineralization or bone formation. Similar results were reported in Meikle et al., Biomaterials, (1994) 15(7):513-521 with poly DL-lactide-co-glycolide discs having bone matrix extract incorporated therein.
U.S. Pat. No. 4,563,350 describes a composition suitable for inductive bone implants comprising a purified form of osteogenic factor in admixture with a carrier having a percentage of non-fibrillar collagen. The factor is added to the collagen either in solution or gelatin form and stirred in dilute mineral acid for 1-2 hours at approximately 4.degree. C. The material is then dialyzed and lyophilized.
Japanese Laid-Open Patent Publication No. 5253286 describes a bone restoring material comprising Ca-containing glass powder and or crystallized glass powder, an aqueous solution composed mainly of phosphate, and a medical substance in release-controlled form. The medical substance is described as being in particulate form and can be coated with materials capable of oppressing the releasing of the substance temporarily.
As can be seen from the foregoing, a carrier providing for the controlled release of biologically active molecules is needed. Such materials which are additionally osteoconductive and/or osteoinductive are also needed.
Bioactive glasses are osteoconductive but are usually formed by combining the different oxides in a platinum crucible and melting the mixture at a temperature of 1300.degree.-1400.degree. C. This is the melt-derived, or conventional, method of obtaining bioactive glasses. Such temperatures, however, would destroy the function of most biologically active molecules during preparation.
Another method which can be used to synthesize bioactive glass is that of sol-gel processing. Sol-gel synthesis of glasses is achieved by combining a metal alkoxide precursor, such as tetraethylorthosilane (TEOS, Si(OC.sub.2 H.sub.5).sub.4 in the case of silica), with water and an acid catalyst to produce a hydrolysis reaction with consequent polymerization of the metal alkoxide species and production of a gel. This gel will consist mostly of the metal oxide when dried and will attain the consistency of glass.
Several investigators have reported the incorporation of proteins into a sol-gel-type glass produced using silicon alkoxide precursors and water with a maintenance of function. Braun et al., J. of Non-Crystalline Solids, (1992) 147 and 148:739-743; Yamanaka et al., Chemistry of Materials, (1992) 4(3):495-497; Ellerby et al., Science, (1992) 255:1113-1115; and Avnir et al., Encapsulation of Organic Molecules and Enzymes, Ch. 27, pp385-404, American Chemical Society (1992). Methods for synthesizing low temperature, low alcohol, low proton-concentration sol-gels for enzyme incorporation are described. The incorporated proteins maintained their functionality. However, the focus of such procedures was the immobilization of the protein within the sol-gel in a manner which retains the protein of interest within the gel. When the sol-gel material functions as a sensor, very small molecules, such as glucose, can pass through the pores for assay. The incorporation within the sol-gel provides for repeated use of the protein. Release of the protein from the sol-gel was not desired and would actually be counter to maintenance of long-term activity.
In U.S. Pat. No. 5,074,916, alkali-free bioactive sol-gel compositions based on SiO.sub.2, CaO, and P.sub.2 O.sub.5 are described. Compositions ranges are 44-86, 4-46, and 3-15 weight percent, respectively. However, the process described utilizes temperatures around 600.degree.-800.degree. C. Such a process is totally incompatible with the incorporation of biological molecules.